The invention relates to a method and a device for taking an image of an object surface by means of a focusable radiation, in particular, ultrasound. Within the scope of the invention, the aim is to generate an image of a two-dimensional surface with the aid of simple means.
The dissertation entitled xe2x80x9cTheoretische und experimentelle Untersuchungen zur quasioptischen Abbildung mit einer Ultraschall-Transmissionskameraxe2x80x9d[xe2x80x9cTheoretical and experimental investigations into quasi-optical imaging with the aid of an ultrasonic transmission cameraxe2x80x9d] by Ralph Oppelt, Friedrich Alexander University of Erlangen-Nurnberg, Germany, 1985, pages 21-32, discloses a device (xe2x80x9cthe Oppelt devicexe2x80x9d) for taking an image of an object surface by means of ultrasound which has an unfocused ultrasound transducer as transmitter for the radiation, and a two-dimensional arrangement having many ultrasound transducers as receiver. A corresponding method for taking an image by using this device is also apparent from the dissertation. A substantial disadvantage of this prior art device is the high level of complexity of the receiver. Each transducer forms an array element of the image to be taken, and therefore the number of the transducers must be extremely high, in particular many thousands, to obtain a sufficiently sharp image. The device for evaluating the signals of all the transducers would also require a correspondingly complicated design.
In the article entitled xe2x80x9cCrossed-Transducers Array for Transmission Ultrasonic Imagingxe2x80x9d by M. Bernard, IEEE 1983 Ultrasonics Symp., page 732, a device is disclosed which provides a linear arrangement of many rod-shaped transducers both for the transmitter and for the receiver. The arrangement of the receiver is aligned orthogonally to the arrangement of the transmitter. Each transducer of the transmitter generates a signal extending in the shape of a strip, and each transducer of the receiver is sensitive to a strip-shaped signal. An array for the image to be taken is defined by the crossing points of the strip-shaped signals respectively of a transducer in the transmitter, and respectively of a transducer in the receiver. An essential feature of this device is that the number of the transducers is substantially reduced, in contrast to the Oppelt device, however, this device is suitable exclusively for taking an image in the near field region of each transducer used. This results in the device being unsuitable in principle for taking an image of a sharpness desired for applications, such as medical imaging or in nondestructive testing. The device is also unsuitable for taking an image of an object surface which is located in the interior of an object that is extended relative to the wavelength of the ultrasound used.
DE 26 55 274 A1 also discloses a method and a device for taking an image of an object surface by means of ultrasound. In this device, a two-dimensional arrangement with many ultrasound transducers serves both as transmitter and as receiver. The individual ultrasound transducers are combined to form a plurality of line groups in the transmit mode and in the receive mode. The line groups in the transmit mode are orthogonal to the line groups in the receive mode. The disclosed device is intended in this case exclusively for a reflection mode, and it is not possible to transradiate the object surface to be imaged which is necessary for a transmission mode.
The object of the present invention is to provide a simple method and device for imaging an object surface by means of focusable radiation, e.g., ultrasound which results in an image having high quality precision and sharpness. Furthermore, the method and device of the present invention are intended to be suitable both for a reflection mode and for a transmission mode. Such a method comprises the following steps:
a) defining an array, covering the surface of the object elements;
b) for each object element:
1) using a transmitter to emit a first signal of the radiation that is focused onto an associated transmitter line, including the object element, in the object surface;
2) using a receiver constructed separately from the transmitter to receive a second signal of the radiation that is focused onto an associated receiver line, including the object element and crossing the associated transmitter line, in the object surface;
3) picking-up an associated image information item for the object element from the second signal; and
c) composing the image from the image information items of all the object elements.
The foregoing method is achieved by using a device for taking an image of an object surface by means of a focusable radiation, which comprises the following components:
a) means for defining an array of object elements that covers the object surface;
b) a transmitter, which is set up in order to emit for each object element a first signal of the radiation that is focused onto an associated transmitter line, including the object element, in the object surface;
c) a receiver, constructed separately from the transmitter, which is set up in order to receive for each object element a second signal of the radiation that is generated by means of the first signal and is focused onto an associated receiver line, including the object element and crossing the associated transmitter line, in the object surface;
d) a co-ordination device for controlling the transmitter and the receiver in such a way that for each object element a first signal is emitted and a second signal is received, and an associated image information item is picked up from each second signal; and
e) a composing device for composing the image from the image information items of all the object elements.
Accordingly, the present invention requires an array of object elements to be defined in the object surface the image of which is to be taken. This array is provided by crossing points of transmitter lines and receiver lines. A transmitter is used which is set up in order optionally to generate a first signal focused onto any desired transmitter line, and a receiver is used which is set up in order optionally to receive a second signal focused onto each receiver line. The transmitter and receiver are separate units. The selection of a pixel in relation to which an image information item is to be picked up is performed by selecting the associated transmitter line and the associated receiver line, emitting the corresponding first signal, and receiving the corresponding second signal. Since the transmitter line and the receiver line cross one another only in the corresponding object element, the image information item derived from the second signal corresponds to the desired image information item of the object element.
It is possible by using a focusing transmitter and a focusing receiver to determine the object surface freely and to operate outside the near field regions of the transmitter and the receiver. This removes any corresponding restriction, thereby making it possible to use a virtually controllably fine array for the object surface, and thus to generate an image of the object surface down to a fineness limited only by the wavelength of the radiation used. Any restriction regarding the extension of an object in which the object surface lies is also removed. It is therefore possible to produce a sectional image of a real object, in particular a living body in the field of medical imaging technology, or of a technical object within the field of nondestructive material testing.
It is necessary to ensure in each application that the transmitter and the receiver are suitably coupled to an object to be investigated. The structural separation of transmitter and receiver permits a very wide possibility of uses. Further, depending on the positioning of transmitter and receiver (with reference to the object surface), a reflection mode or a transmission mode is possible.
In a preferred embodiment of the novel method and the device, each receiver line crosses each transmitter line orthogonally. This enables an array resulting therefrom to be a conventionally orthogonal array.
In a further preferred embodiment of the present invention the object surface is flat, in which case the transmitter and the receiver may require a particular configuration which is disclosed hereinbelow in the context of an exemplary embodiment.
In accordance with the present invention, the second signal is preferably generated by transmission of the first signal through the object surface, or by reflection of the first signal at the object surface. Both transmission and reflection can be performed with or without deflection of the first signal. Selection of the deflection may be suitable for obtaining specific information on the nature of the image surface or of the object including the image surface.
Ultrasound is preferred as the focusable radiation. An application of light, in particular laser light, and other electromagnetic radiation is, however, not excluded.
The means provided in the device for defining an array covering the surface of the object elements can be formed in various ways. A particularly preferred means in the form of a map installed in the co-ordination device. This map can select from a plurality of transducers in the transmitter and/or in the receiver, a transducer which corresponds to a prescribed transmitter line or a prescribed receiver line. The map can also serve the purpose of setting a predetermined position for a single transducer in the transmitter and/or a single transducer in the receiver, in accordance with the selected transmitter line or selected receiver line, respectively. The setting can be a mechanical displacement, or a direction effected by means of appropriate electronic control. It is also conceivable to provide the means as a combination of a map, installed in the co-ordination device, with a mechanical device, for example an arrangement of mechanical latches in the transmitter or in the receiver.
In a preferred embodiment, the transmitter or the receiver of the device comprises for each transmitter line or for each receiver line an associated transducer and an associated focusing means. The transducer can be of punctiform design and the focusing means of astigmatic design in order to achieve the desired linear focusing onto a transmitter line or a receiver line. Alternatively, it is preferred for each associated transducer to be of linear design, and each associated focusing means of cylindrical design. Focusing means include a lens, and depending on the application, specifically a cylindrical or astigmatic lens, or a cylindrical singly curved, or astigmatic double, in particular an elliptically curved mirror.
In a further preferred embodiment of the novel device the transmitter or the receiver is a phased array of transducers. Such a transmitter is distinguished in that the first signal it emits can be focused optionally onto any transmitter line with the aid of electronic means, without the need for mechanical intervention. The same is true for a corresponding receiver.